Goal
The objective of this project is to develop and demonstrate a technology that uses vertical-force seismic sources combined with vertical seismic profile (VSP) to provide a seismic "log" of natural fracture orientation and density in unconventional reservoirs.

Performers
University of Texas, Bureau of Economic Geology, Austin, TX 78713-8924
GEDCO - an integrated geophysical survey design software and services company that is part of Schlumberger’s WesternGeco business unit

Background
A common feature of shale-gas units and tight sandstones is that most of these unconventional reservoirs have embedded fracture systems that need to be understood in order to position exploitation wells. A remote seismic technology that can “visualize” the internal architecture of unconventional reservoirs and predict fracture orientation and density will be invaluable for characterizing tight sandstones, shale-gas units, and all unconventional resource plays. Current technology has demonstrated that shear (S) waves are more responsive to fractures than compressional (P) waves. Based on this knowledge, operators across unconventional reservoir plays need an effective, low-cost way to illuminate reservoir systems with surface-generated S waves. This project will develop a technology whereby S waves can be produced with simple, low-cost, and widely available seismic sources that apply a vertical force to the Earth. The technology utilizes S modes created directly at the point where a vertical force is applied to the Earth’s surface, which is in contrast to current practices of using sources that apply a horizontal force to the Earth or converted S modes produced at subsurface interfaces by downgoing P wavefields.

In addition, researchers on this project will present field procedures and data-processing strategies whereby S modes produced by vertical-force sources can provide fracture sensitive attributes. These procedures remove current practice limitations because vertical-force sources (vertical vibrators, vertical impacts, shothole explosives) are lower cost, more abundant than horizontal-force sources, and usable over a wider range of terrains. This approach refutes the common assumption that the only way to create a downgoing vertical shear (SV) mode with a vertical-force source is to use a shallow interface to produce a downgoing P-to-SV mode conversion. An important feature of the technology is that fracture properties can be estimated several tens of meters away from a receiver well rather than the one meter distance required for a dipole sonic log.

The outputs of the project will be a demonstration of correct field procedures for acquiring orthogonal SV-wave vectors and software source code that performs data analyses to convert orthogonal SV-wave data into estimates of fracture orientation and fracture density.

Potential Impacts
The seismic technology developed in this study will allow improved mapping of fracture orientations and densities in unconventional reservoirs, thus addressing the objective of advanced visualization to enhance unconventional production. The technology was developed using VSP data acquired in shale-gas and tight-sandstone reservoirs, but it can be applied to any fractured reservoir. Providing operators with better knowledge of fracture density and crack orientation should result in increased oil production.

Simplifying S-wave seismic source activity by utilizing S modes created directly at the point where a vertical force is applied to the Earth’s surface will result in less costly data acquisition and fewer environmental issues caused by source deployment. An important impact is that vertical-force seismic sources can be utilized in a wide variety of terrains where horizontal-force sources cannot be deployed.

Accomplishments
VSP data that illuminate unconventional reservoirs in two diverse geological settings have been analyzed to demonstrate that the use of direct-S modes produced by vertical vibrators at orthogonal-azimuth source stations is a valuable way to evaluate fracture systems and anisotropic properties of unconventional reservoir systems.

This study documents Alford rotation, which can be applied to a wider range of VSP data than some investigators have supposed. Examples from this study show Alford rotation concepts can be applied to VSP data that violate the various assumptions that many investigators impose on VSP data that are used to determine natural-coordinate axes.

In the VSP application, it is preferred to deploy vertical vibrators at source stations that have source-to-receiver azimuths that differ by 90° in order to illuminate a fractured interval with orthogonal S-wave displacement vectors. However, this orthogonal-azimuth source-station geometry has to be abandoned when acquiring 2-D and 3-D reflection data with surface receivers.

VSP data can be used when source-receiver offsets exceed receiver depth, and when raypath arrival angles are 45° and larger. This use of Alford rotation contrasts with the common assumptions that source-to-receiver offsets for data used in Alford rotation procedures should be small and that raypath arrival angles of S-wave raypaths at VSP receiver stations should be close to vertical.

VSP S-wave data can be used when the radial and transverse S-wave displacement vectors used in Alford rotation are not oriented in orthogonal azimuths. Most data processors take great care to use S-wave data in Alford rotation analyses in which radial and transverse displacement vectors are as close to orthogonal as possible.

Vertical vibrators used in this analysis were positioned at widely separated source stations and raypaths from the sources traversed significantly different overburden conditions before reaching a targeted reservoir interval. In contrast, data preferred for Alford rotation are produced by orthogonal horizontal vibrators positioned at the same zero-offset source station, so that source wavelets travel identical trajectory paths through identical overburden conditions to reach a fracture interval.

SV shear wavefields are produced directly at the point where a vertical vibrator applies its vertical force vector to the earth. These direct-SV wavefields are robust and can be used to estimate S-wave anisotropy in the same manner as S wavefields produced by horizontal vibrators.

When vertical-vibrator source stations are distributed in a circle around a VSP well, azimuth-dependent direct-S and direct-P average velocities can be calculated to allow natural-coordinate axes to be recognized. Using all available calibration data, the azimuths of these natural coordinate axes can then be associated with either the azimuths of vertical fractures or with the azimuths of maximum and minimum horizontal stresses.

One important consideration is that the cost of acquiring multicomponent seismic data can be reduced by using vertical-force sources to generate direct-S waves. Because of the potential commercial value of using vertical-force sources to generate direct-S modes, the concepts illustrated in this research have been patented by the Board of Regents of The University of Texas System (Hardage, 2011).

Vertical vibrators are widespread and horizontal vibrators are not, a second implication is that direct-S data acquisition can be considered across many areas where S-wave technology could not otherwise be performed.

Current Status (June 2013)
All project work has been completed. The final report is being reviewed.